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Transcript of Paul Ashall, 2007 Membrane processes. Paul Ashall, 2007 Membrane processes Microfiltration...
Paul Ashall, 2007
Membrane processes
Paul Ashall, 2007
Membrane processes• Microfiltration
• Ultrafiltration
• Reverse osmosis
• Gas separation/permeation
• Pervaporation
• Dialysis
• Electrodialysis
• Liquid membranes
• Etc
Paul Ashall, 2007
Membrane applications in the pharmaceutical industry
• UP water (RO)
• Nitrogen from air
• Controlled drug delivery
• Dehydration of solvents
• Waste water treatment
• Separation of isomers (e.g. naproxen) (‘Membrane Technology and Applications’ pp517, 518)
• Membrane extraction
• Sterile filtration
Paul Ashall, 2007
Specific industrial applications
Dialysis – hemodialysis (removal of waste metabolites, excess body water and restoration of electrolyte balance in blood)
Microfiltration – sterilization of pharmaceuticals; purification of antibiotics;separation of mammalian cells from a liquid
Ultrafiltration – recovery of vaccines and antibiotics from fermentation broth
etc
Ref. Seader p715
Paul Ashall, 2007
FEED
RETENTATE
PERMEATE
Paul Ashall, 2007
• Membrane structure (dense, microporous, asymmetric, composite, membrane support)
Paul Ashall, 2007
Membrane types - isotropic
• Microporous – pores 0.01 to 10 microns diam.; separation of solutes is a function of molecular size and pore size distribution
• Dense non-porous – driving force; diffusion; solubility
• Electrically charged microporous
Paul Ashall, 2007
Anisotropic (asymmetric)
• Thin active surface layer supported on thicker porous layer
• Composite – different polymers in layers
• Others – ceramic, metal, liquid
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Asymmetric membranes
Thin dense layer
Microporous support
Paul Ashall, 2007
Membrane materials
• Polymers
• Metal membranes
• Ceramic membranes (metal oxide, carbon, glass)
• Liquid membranes
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Membrane fabrication
Isotropic
• Solution casting
• Melt extrusion
• Track etch membranes (Baker fig. 3.4)
• Expanded film membranes (Baker fig. 3.5)
Paul Ashall, 2007
continued
Anisotropic
• Phase separation (Loeb – Sourirajan method) (see Baker fig. 3.12)
• Interfacial polymerisation
• Solution coated composite membranes
• Plasma deposition
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Membrane modules
• Plate and frame - flat sheets stacked into an element
• Tubular (tubes)• Spiral wound designs using flat sheets• Hollow fibre - down to 40 microns diam. and
possibly several metres long ; active layer on outside and a bundle with thousands of closely packed fibres is sealed in a cylinder
Paul Ashall, 2007
Paul Ashall, 2007
Spiral wound module
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Membrane filtration – Buss-SMS-Canzler
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Operating considerations
• Membrane fouling• Concentration polarisation (the layer of solution
immediately adjacent to the membrane surface becomes depleted in the permeating solute on the feed side of the membrane and enriched in this component on the permeate side, which reduces the permeating components concentration difference across the membrane, thereby lowering the flux and the membrane selectivity)
• Flow mode (cross flow, co-flow, counter flow)
Paul Ashall, 2007
Aspects• Crossflow (as opposed to ‘dead end’) – cross
flow velocity is an important operating parameter
• Sub-micron particles
• Thermodynamic driving force (P, T, c etc) for transport through membrane is activity gradient in membrane
• Flux (kg m-2 h-1)
• Selectivity
• Membrane area
Paul Ashall, 2007
Characteristics of filtration processes
Process technology
Separation principle
Size range MWCO
MF Size 0.1-1μm -
UF Size,charge 1nm-100nm >1000
NF Size, charge, affinity
1nm 200-1000
RO Size, charge, affinity
< 1nm <200
Paul Ashall, 2007
Process technology
Typical operating pressure (bar)
Feed recovery (%)
Rejected species
MF 0.5-2 90-99.99 Bacteria, cysts, spores
UF 1-5 80-98 Proteins, viruses, endotoxins, pyrogens
NF 3-15 50-95 Sugars, pesticides
RO 10-60 30-90 Salts, sugars
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Models
• Ficks law (solution-diffusion model)
Free volume elements (pores) are spaces between polymer chains caused by thermal motion of polymer molecules.
• Darcys law (pore flow model)
Pores are large and fixed and connected.
Paul Ashall, 2007
Simple model (liquid flow through a pore using Poiseuilles
law)J = Δp ε d2
32 μ lJ = fluxl = pore lengthd = pore diam.Δp = pressure difference across pore μ = liquid viscosityε = porosity (π d2 N/4, where N is number of pores per cm2)J/Δp – permeance
Typical pore diameter: MF – 1micron; UF – 0.01 micron
Paul Ashall, 2007
Mechanisms for transport through membranes
• Bulk flow
• Diffusion
• Solution-diffusion (dense membranes – RO, PV, gas permeation)
Paul Ashall, 2007
continued
• Dense membranes: transport by a solution-diffusion mechanism
• Microporous membranes: pores interconnected
Paul Ashall, 2007
Separation of liquids
• Porous membranes
• Asymmetric membranes/dense polymer membranes
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continued• With porous membranes separation may depend just
on differences in diffusivity.• With dense membranes permeation of liquids occurs
by a solution-diffusion mechanism. Selectivity depends on the solubility ratio as well as the diffusivity ratio and these ratios are dependent on the chemical structure of the polymer and the liquids. The driving force for transport is the activity gradient in the membrane, but in contrast to gas separation, the driving force cannot be changed over a wide range by increasing the upstream pressure, since pressure has little effect on activity in the liquid phase.
Paul Ashall, 2007
Microporous membranes
• Porosity (ε)• Tortuosity (τ) (measure of path length compared
to pore diameter)• Pore diameter (d)
Ref. Baker p68 – Fig 2.30
Paul Ashall, 2007
Microporous membranes
• Screen filters (see Baker fig. 2.31) – separation of particles at membrane surface.
• Depth filters (see Baker fig. 2.34) – separation of particles in interior of the membrane by a capture mechanism; mechanisms are sieving and adsorption (inertial capture, Brownian diffusion, electrostatic adsorption)
Ref. Baker pp69, 73
Paul Ashall, 2007
Filtration
• Microfiltration (bacteria – potable water, 0.5 – 5 microns). Pore size specified.
• Ultrafiltration (macromolecules, molecular mass 1000 – 106, 0.5 – 10-3 microns). Cut-off mol. wt. specified.
• Nanofiltration (low molecular weight, non-volatile organics from water e.g. sugars). Cut off mol. wt. specified.
• Reverse osmosis (salts)
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continued
Crossflow operation (as opposed to ‘dead end’ filtration)
Paul Ashall, 2007
Membrane types
• Dense
• High porosity
• Narrow pore size distribution
Paul Ashall, 2007
Ultrafiltration(UF)Uses a finely porous membrane to separate water and
microsolutes from macromolecules and colloids.Membrane pore diameter 0.001 – 0.1 μm.Nominal ‘cut off’ molecular weight rating assigned to
membrane.Membrane performance affected by:• Concentration polarisation• Membrane fouling• Membrane cleaning• Operating pressure
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Spiral wound UF module
Paul Ashall, 2007
UF
Membrane materials (Loeb- Sourirajan process)• Polyacrylonitrile (PAN)• PVC/PAN copolymers• Polysulphone• PVDF (polyvinylidene difluoride)• PES (polyethersulfone)• Cellulose acetate (CA)
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UF
Modules
• Tubular
• Plate and frame
• Spiral wound
• Capillary hollow fibre
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UF applications
• Protein concentration
Paul Ashall, 2007
Microfiltration (MF)
Porous membrane; particle diameter 0.1 – 10 μm
Microfiltration lies between UF and conventional filtration.
In-line or crossflow operation.
Screen filters/depth filters (see Baker fig. 7.3, p 279)
Challenge tests developed for pore diameter and pore size.
Paul Ashall, 2007
MF
Membrane materials
• Cellulose acetate/cellulose nitrate
• PAN – PVC
• PVDF
• PS
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MF
Modules
• Plate and frame
• Cartridge filters (see Baker figs. 7.11/7.13, p288, 290)
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MF operation
• Fouling
• Backflushing
• Constant flux operation
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MF uses
• Sterile filtration of pharmaceuticals (0.22 μm rated filter)
• Drinking water treatment
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Reverse osmosisMiscible solutions of different concentration separated
by a membrane that is permeable to solvent but impermeable to solute. Diffusion of solvent occurs from less concentrated to a more concentrated solution where solvent activity is lower (osmosis).
Osmotic pressure is pressure required to equalise solvent activities.
If P > osmotic pressure is applied to more concentrated solution, solvent will diffuse from concentrated solution to dilute solution through membrane (reverse osmosis).
Paul Ashall, 2007
Reverse osmosis
The permeate is nearly pure water at ~ 1atm. and very high pressure is applied to the feed solution to make the activity of the water slightly greater than that in the permeate. This provides an activity gradient across the membrane even though the concentration of water in the product is higher than that in the feed.
Paul Ashall, 2007
Reverse osmosis
Permeate is pure water at 1 atm. and room temperature and feed solution is at high P.
No phase change.Polymeric membranes used e.g. cellulose
acetate20 – 50 atm. operating pressure.Concentration polarisation at membrane
surface.
Paul Ashall, 2007
RO
F
R
PP1 P2
P1 » P2
Paul Ashall, 2007
Model
• Flux equations
• Salt rejection coefficient
Paul Ashall, 2007
Water flux
Jw = cwDwvw (ΔP – Δπ) RT z
Dw is diffusivity in membrane, cm2 s-1
cw is average water conc. in membrane, g cm-3 (~ 0.2)
vw is partial molar volume of water, cm3g-1
ΔP pressure differenceR gas constantT temperatureΔπ osmotic pressurez membrane thickness
Paul Ashall, 2007
Salt flux
Js = Ds Ss (Δcs)
z
Ds diffusivity
Ss solubility coefficient
Δcs difference in solution concentration
Ref. Baker pp 34, 195
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Jw increases with ΔP and selectivity increases also since Js does not depend on ΔP.
Paul Ashall, 2007
Membrane materials
• Asymmetric cellulose acetate• Polyamides• Sulphonated polysulphones• Substituted PVA• Interfacial composite membranes• Composite membranes• Nanofiltration membranes (lower pressure, lower
rejection; used for lower feed solution concentrations)
Ref. Baker p203
Paul Ashall, 2007
RO modules
• Hollow fibre modules (skin on outside, bundle in sealed metal cylinder and water collected from fibre lumens; individual fibres characterised by outside and inside diameters)
• Spiral wound modules (flat sheets with porous spacer sheets, through which product drains, and sealed edges; a plastic screen is placed on top as a feed distributor and ‘sandwich’ is rolled in a spiral around a small perforated drain pipe) (see McCabe fig. 26.19)
• Tubular membranes
Paul Ashall, 2007
Operational issues
• Membrane fouling• Pre-treatment of feed solutions• Membrane cleaning• Concentration polarisation (higher conc. of solute at
membrane surface than in bulk solution – reduces water flux because the increase in osmotic pressure reduces driving force for water transport and solute rejection decreases because of lower water flux and greater salt conc. at membrane surface increases solute flux) (Baker ch. 4)
• > 99% salt rejection
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Example
See McCabe p893
Paul Ashall, 2007
Applications
• UP water (spec. Baker pp 226, 227)
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Dialysis
A process for selectively removing low mol. wt. solutes from solution by allowing them to diffuse into a region of lower concentration through thin porous membranes. There is little or no pressure difference across the membrane and the flux of each solute is proportional to the concentration difference. Solutes of high mol. wt. are mostly retained in the feed solution, because their diffusivity is low and because diffusion in small pores is greatly hindered when the molecules are almost as large as the pores.
Uses thin porous membranes.
Paul Ashall, 2007
Electrodialysis
Ions removed using ion selective membranes across which an electric field is applied.
Used to produce potable water from brackish water. Uses an array of alternate cation and anion permeable membranes.
Paul Ashall, 2007
Pervaporation (PV)
In pervaporation, one side of the dense membrane is exposed to the feed liquid at atmospheric pressure and vacuum is used to form a vapour phase on the permeate side. This lowers the partial pressure of the permeating species and provides an activity driving force for permeation.
Paul Ashall, 2007
PV
The phase change occurs in the membrane and the heat of vapourisation is supplied by the sensible heat of the liquid conducted through the thin dense layer. The decrease in temperature of the liquid as it passes through the separator lowers the rate of permeation and this usually limits the application of PV to removal of small amounts of feed, typically 2 to 5 % for 1-stage separation. If a greater removal is needed, several stages are used in series with intermediate heaters.
Paul Ashall, 2007
Pervaporation (PV)
• Hydrophilic membranes (PVA) e.g. ethanol/water
• Hydrophobic membranes (organophilic) e.g. PDMS
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PV
• Composite membrane (dense layer + porous supporting layer)
Ref. Baker p366
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Modules
• Plate & frame (Sulzer/GFT)
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PV
• Solution –diffusion mechanism
• Selectivity dependent on chemical structure of polymer and liquids
Paul Ashall, 2007
PV
Activity driving force is provided by difference in pressure between feed and permeate side of membrane.
Component flux is proportional to concentration and diffusivity in dense membrane layer.
Flux is inversely proportional to membrane thickness.
Paul Ashall, 2007
Models
• Solution – diffusion model
• Experimental evidence (ref. Baker pp 43 – 48)
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continued
Ji = PiG (pio – pil)
l
Ji – flux, g/cm2s
PiG – gas separation permeability coefficient, gcm. cm-2 s-1. cmHg-1
l – membrane thickness
pio – partial v.p. i on feed side of membrane
pil – partial vp i on permeate side
Paul Ashall, 2007
PV selectivity
β = (cil/cjl)
(cio/cjo)
cio conc. i on feed side of membrane
cil conc. i on permeate side of membrane
cjo conc. j on feed side
cjl conc. j on permeate side
Paul Ashall, 2007
continued
Structure – permeability relationships• Sorption coefficient, K (relates
concentration in fluid phase and membrane polymer phase)
• Diffusion coefficient, D
Ref. Baker p48
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continued
Diffusion in polymers
• Glass transition temperature,Tg
• Molecular weight, Mr
• Polymer type and chemical structure,
• Membrane swelling,
• Free volume correlations
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continued
Sorption coefficients in polymers vary much less than diffusion coefficients, D.
nim = pi/pisat , where nim is mole fraction i absorbed, pi is partial pressure of gas and pisat is saturation vapour pressure at pressure and temperature of liquid.
Vi = pi/pisat , where Vi is volume fraction of gas 2.72 absorbed by an ideal polymer
Paul Ashall, 2007
Dual sorption model
Gas sorption in a polymer occurs in two types of site (equilibrium free volume and excess free volume (glassy polymers only)).
Baker pp56-58
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continued
Flux through a dense polymer is inversely proportional to membrane thickness.
Flux generally increases with temperature (J = Jo exp (-E/RT).
An increase in temperature generally decreases membrane selectivity.
Paul Ashall, 2007
PV process design
• Vacuum driven process• Condenser• Liquid feed has low conc. of more permeable
species
Ref. Baker p 370
Paul Ashall, 2007
Applications
• Dehydration of solvents e.g. ethanol (see McCabe pp886-889, fig. 26.16/example 26.3)
• Water purification/dissolved organics e.g. low conc. VOC in water with limited solubility
• Organic/organic separations
Paul Ashall, 2007
PV – hybrid processes using distillation
Paul Ashall, 2007
continued
• Measures of selectivity• Rate (flux, membrane area)• Solution –diffusion model in polymeric
membranes (RO, PV etc)• Concentration polarisation at membrane
surface• Membrane fouling• Batch or continuous operation
Paul Ashall, 2007
Gas separation
When a gas mixture diffuses through a porous membrane to a region of lower pressure, the gas permeating the membrane is enriched in the lower mol. wt. component(s), since they diffuse more rapidly.
Paul Ashall, 2007
Gas separation
The transport of gases through dense (non-porous) polymer membranes occurs by a solution-diffusion mechanism.The gas is absorbed in the polymer at the high pressure side of the membrane, diffuses through the polymer phase and desorbs at the low pressure side. The diffusivities in the membrane depend more strongly on the size and shape of the molecules than do gas phase diffusivities.
Paul Ashall, 2007
continued
Gas separation processes operate with pressure differences of 1 – 20 atm., so the thin membrane must be supported by a porous structure capable of withstanding such pressures but offering little resistance to the flow of gas. Special methods of casting are used to prepare asymmetric membranes, which have a thin, dense layer or ‘skin’ on one side and a highly porous substructure over the rest of the membrane. Typical asymmetric membranes are 50 to 200 microns thick with a 0.1 to 1 micron dense layer.
Paul Ashall, 2007
Mechanisms
• Convective flow (large pore size 0.1 – 10 μm; no separation)
• Knudsen diffusion (pore size < 0.1μm; flux α 1/(Mr)1/2)
• Molecular sieving (0.0005 – 0.002 μm)• Solution-diffusion (dense membranes)
(See Baker fig. 8.2, p303)
Paul Ashall, 2007
Knudsen diffusion
Knudsen diffusion occurs when the ratio of the pore radius to the gas mean free path (λ ~ 0.1 micron) is less than 1. Diffusing gas molecules then have more collisions with the pore walls than with other gas molecules. Gases with high D permeate preferentially.
Paul Ashall, 2007
Poiseuille flow
If the pores of a microporous membrane are 0.1 micron or larger, gas flow takes place by normal convective flow.i.e. r/λ > 1
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Transport of gases through dense membranes
JA = QA (pA1 – pA2)
QA is permeability (L (stp) m-2 h-1 atm-1)
pA1 partial pressure A feed
pA2 partial pressure A permeate
Paul Ashall, 2007
Membrane selectivity
α = QA/QB = DASA/DBSB
D is diffusion coefficient
S is solubility coefficient (mol cm-3 atm-1) i.e. cA = pASA, cB = pBSB
(Ref. McCabe ch. 26 pp859 – 860)
Paul Ashall, 2007
Diffusion coefficients in PET (x 109 at 25oC, cm2 s-1)
Polymer O2 N2 CO2 CH4
PET 3.6 1.4 0.54 0.17
Paul Ashall, 2007
Membrane materials
• Metal (Pd – Ag alloys/Johnson Matthey for UP hydrogen)
• Polymers (typical asymmetric membranes are 50 to 200 microns thick with a 0.1 to 1 micron skin)
• Ceramic/zeolite
Paul Ashall, 2007
Modules
• Spiral wound
• Hollow fibre
Paul Ashall, 2007
Flow patterns
• Counter-current
• Co-/counter
• Radial flow
• crossflow
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System design
• Feed/permeate pressure (Δp = 1 – 20 atm.)
• Degree of separation
• Multistep operation
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Applications
• Oxygen/nitrogen separation from air (95 – 99% nitrogen)
• Dehydration of air/air drying
Ref. Baker p350
Paul Ashall, 2007
Other membrane processes
• Ion exchange
• Electrodialysis e.g. UP water
• Liquid membranes/carrier facilitated transport e.g. metal recovery from aqueous solutions
Paul Ashall, 2007
PV demonstration
Paul Ashall, 2007
Reference texts
• Membrane Technology and Applications, R. W. Baker, 2nd edition, John Wiley, 2004
• Handbook of Industrial Membranes, Elsevier, 1995• Unit Operations in Chemical Engineering ch. 26, W.
McCabe, J. Smith and P. Harriot, McGraw-Hill, 6th edition, 2001
• Transport Processes and Unit Operations, C. J. Geankoplis, Prentice-Hall, 3rd edition, 1993
• Membrane Processes: A Technology Guide, P. T. Cardew and M. S. Le, RSC, 1998
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continued
• Perry’s Chemical Engineers’ Handbook, 7th edition, R. H. Perry and D. W. Green, McGraw-Hill, 1998
• Separation Process Principles, J. D. Seader and E. J. Henley, John Wiley, 1998
• Membrane Technology in the Chemical Industry, S. P. Nunes and K. V. Peinemann (Eds.), Wiley-VCH, 2001